1. Field of the Invention
This invention relates to grafting methods for vascular bypass surgery and, more particularly, to such methods which utilize a prosthetic graft.
2. Description of the Prior Art
A common medical problem, particularly for patients advancing in age, is atherosclerosis or hardening of the arteries. Blockage develops in the artery and restricts circulation of blood to limbs and organs. In the legs, for example, such blockage may result in gangrene and subsequent amputation of the leg. Treatment of this condition is frequently accomplished with arterial bypass surgery in which a graft is affixed to the afflicted artery to form a new flow passage around the area of blockage. In this manner, circulation to the limb or organ may be reestablished.
Initially, the practitioner must choose between using a natural blood vessel or using a prosthetic blood vessel in constructing the bypass graft. Natural blood vessel grafts, such as vein grafts, are generally preferred over prosthetic grafts because of their better long-term patency rate. Prosthetic grafts are normally used only when vein grafts are not available or not suitable. The saphenous vein, which runs under the skin from the ankle up to the groin, is often used as a natural arterial bypass graft. Referring to FIG. 1, in a common procedure known as a femoral-popliteal bypass, the saphenous vein is removed and a portion A is excised for grafting directly onto a femoral artery B and a popliteal artery C to provide for blood flow around a blockage D intermediate of the femoral and popliteal arteries.
Vein grafts, although considered the replacement of choice for reconstruction of arteries located below the groin, are known to be susceptible to structural changes after their implantation. The following structural changes have been observed in vein grafts, each possibly leading to loss of patency: (1) intimal hyperplasia-excessive thickening of the inner layer, or intima, of the blood vessel; this process may involve the anastomosis with the host artery, may be a focal lesion found anywhere along the vein graft or may be diffuse, affecting the entire graft; approximately 50% of graft failures occur due to intimal hyperplasia; (2) fibrotic thickening of the vein wall--probably due to damage to the vein during its removal, preparation and anastomosis, in combination with exposure to the high pressure arterial environment; (3) degeneration and fibrosis of the valves contained within the vein graft; (4) aneurysmal dilation of the vein graft due to intrinsic structural deficiency; and (5) Atherosclerosis. Approximately 80% of the vein grafts examined two years or more after implantation show evidence of atherosclerosis. Observation after five years has shown that 56% of femoral-popliteal bypasses appear structurally sound and functionally unimpaired. After ten years, only 44% of femoral-popliteal bypasses appear structurally sound and functionally unimpaired. Regarding femoral-tibial vein graft bypasses, it has been shown that a patency rate of 37% after five years can be expected, whereas, after eight years, this decreases to 28%. Szilagyi, et al., Surgery 1979, 86:836-41. DeWeese, in a similar ten year follow-up of femoral-popliteal vein grafts, reported 59% patency rate at five years and 38% at ten years. Surgery 1977, 82:775-84.
In patients in whom a femoral-popliteal vein bypass fails, a femoral-tibial bypass, i.e., a bypass extending further beyond the popliteal artery C to a tibial artery E, is required as is shown in FIG. 2. Since under today's common procedures, the natural vein graft had already been used for the femoral-popliteal bypass, a prosthetic graft F must be used for a femoral-tibial bypass. The results of a prosthetic femoral-tibial bypass are poor, with only 25% functional after one year.
Prosthetic grafts, when used to reconstruct arteries below the groin, have even a higher failure rate than vein grafts. The estimated five year patency rate for femoral-popliteal bypass is 61.8% using vein grafts and 43.2% using prosthetic grafts. For femoral-tibial bypass procedures, the vein graft shows a 68.4% patency rate, while the prosthetic graft shows a 26.6% patency rate. Femoral-tibial prosthetic bypass grafts generally have a less favorable prognosis as compared to femoral-tibial vein bypass grafts. Br.J. Surgery, 76:7-14, 1989.
Prosthetic grafts are commonly made of Polytetrafluoroethylene (PTFE) Gore-tex.RTM. or Dacron.RTM.. These materials are significantly stiffer or less compliant than the natural arteries to which they are affixed. A difference in mechanical properties, i.e., a compliance mismatch, thus exists at the anastomosis or connection of a prosthetic graft and an artery when the graft is constructed completely from prosthetic material. Referring again to FIG. 2, a constriction (not shown) known as anastomotic intimal hyperplasia frequently develops at the anastomosis between the prosthetic graft F and the artery, here the tibial artery E. This phenomenon is characterized by a proliferation in the production of smooth muscle cells at the anastomosis which causes a narrowing of the anastomosis, a reduction of blood flow, and a subsequent graft failure due to thrombosis. Compliance mismatch between the prosthetic material and the artery is considered to be a significant factor in the genesis of anastomotic intimal hyperplasia.
The problem of matching compliance between a graft and a host artery has long been recognized in cardiovascular surgery. For example, an article in the Oct. 1, 1955 edition of The Lancet, beginning at page 711, states that methods for arterial grafting involving rigid tubes may be expected to fail, particularly in areas of the body where soft tissues are mobile.
The general concept of employing a collar between the graft and the artery has been employed in anastomozing intra- and extra-peritoneal organs. One method utilizes a sealing collar made from a segment of small intestine which is free from mucous, interposing the collar between the organs and the anastomosis suture. Skobelkin, Oct. 24, 1980, page 31.
U.S. Pat. No. 3,409,914 to Jones discloses a connector for blood pumps and the like wherein a clothlike filler material is disposed immediately adjacent the connection between the prosthesis and the artery to permit proper in-growth of tissue. The resultant decrease in the diameter of the connection is said to increase blood flow velocity and provide a washing action at the anastomosis, which reduces the possibility of unsatisfactory clotting and/or tissue growth.
Others in the art have sought to devise a graft which itself sufficiently matches compliance with the natural artery to avoid the undesirable complications discussed above. For example, U.S. Pat. No. 4,098,571 to Miyata et al. discloses a substitute blood vessel and a process for preparing the same wherein a pig blood vessel is pre-treated with a digesting solution to remove all tissue constituents except those which allow the vessel to retain collagenous and elastic fiber qualities. Similarly, the German periodical Intermedicat GMBH discloses in its Dec. 6, 1977 issue a process for stripping blood vessels from human umbilical cords and treating them to produce tubular prostheses.
Attempts have been made more recently to solve the problem of anastomotic intimal hyperplasia resulting from femoral-popliteal bypass surgery. As shown in FIG. 3, a vein collar G is utilized in a femoral-popliteal prosthetic bypass graft by placing the vein collar G between the prosthetic graft H and the popliteal artery C. Since the vein collar G is made from a natural blood vessel, the compliance match between the graft and the popliteal artery C is improved. Vein collar interposition decreases the rate of graft failure, but it does not eliminate the problem. Progressive intimal hyperplasia has been shown to develop in the vein collar itself.
Accordingly, it is an object of the present invention to provide a prosthetic grafting method for bypass surgery which avoids compliance mismatch and at the same time preserves potential graft blood vessels in the patient for subsequent bypass procedures.